HSM Applications to Multilane Rural Highways and Urban Suburban Streets Predicting Crash Frequency and CMFs for Rural Divided Multilane Highways - Session #3 Session #3 – Predicted of Crash Frequency for Divided Rural Multilane Highways

Predicting Crash Frequency and CMFs for Rural Divided Multilane Highways Learning Outcomes: Describe the models to Predict Crash Frequency for Divided Rural Multilane Highways Calculate Predicted Crash Frequency for Divided Rural Multilane Highways Describe Crash Modification Factors Apply Crash Modification Factors Learning Objectives for Session #3 Divided Rural Multilane

Subdividing Roadway Segments Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Subdividing Roadway Segments Before applying the safety prediction methodology to an existing or proposed rural segment facility, the roadway must be divided into analysis units consisting of individual homogeneous roadway segments and intersections. A new analysis section begins at each location where the value of one of the following variables changes (alternatively a section is defined as homogenous if none of these variables changes within the section): • Average daily traffic (ADT) volume (veh/day) • Lane width (ft), Shoulder width (ft), Shoulder Type Side slope • Presence of a median • Major intersections Instructor: From Chapter 11 of the Highway Safety Manual Instructor should review carefully with workshop participants so they have a precise understanding. Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Subdividing Roadway Segments Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Subdividing Roadway Segments Homogeneous roadway segments • Lane width Instructor: From Chapter 11 of the draft Highway Safety Manual HSM provides recommended rounding of lane widths to provide consistency and account for variances Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Subdividing Roadway Segments Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Subdividing Roadway Segments Homogeneous roadway segments • Shoulder width Instructor: From Chapter 11 of the Highway Safety Manual HSM provides recommended rounding of shoulder widths to provide consistency and account for variances Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Subdividing Roadway Segments Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Subdividing Roadway Segments Homogeneous roadway segments • Presence of a median Instructor: From Chapter 11 of the Highway Safety Manual HSM provides recommended rounding of median widths to provide consistency and account for variances. Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Predicting Crash Frequency for an Entire Rural Multilane Segment Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Predicting Crash Frequency for an Entire Rural Multilane Segment Npredicted total = Sum Nrs + Sum Nint Three-step process: Predict number of total roadway segment crashes per year (Nrs) Predict number of total intersection-related crashes per year (Nint) Combine predicted roadway segment and intersection related crashes to obtain the total (Npredicted) Instructor: from Chapter 11 of the Highway Safety Manual The total estimated number of crashes within the network or facility limits during a study period of n years is calculated using Equation 11-5: Where: Ntotal = total expected number of crashes within the limits of a rural two-lane, two-way road facility for the period of interest. Or, the sum of the expected average crash frequency for each year for each site within the defined roadway limits within the study period; Nrs = expected average crash frequency for a roadway segment using the predictive method for one specific year; and Nint = expected average crash frequency for an intersection using the predictive method for one specific year. Equation 11-5 represents the total expected number of crashes estimated to occur during the study period. Equation 11-6 is used to estimate the total expected average crash frequency within the network or facility limits during the study period. Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Predicting Crash Frequency for Rural Multilane Highway Segments Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Predicting Crash Frequency for Rural Multilane Highway Segments Model for Rural Multilane Segments: Nspf rd = e(a + b Ln AADT + Ln L) Where: Nspf rd = Baseline Total Crashes per year for segment L = Length of roadway segment (miles) AADT = Annual Average Daily Traffic (vehicles/day) a & b = regression coefficients Key Message: This slide shows the model for estimating expected crashes for rural multilane From Chapter 11 of the draft highway safety manual. Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Predicting Crash Frequency for Rural Multilane Highway Segments Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Predicting Crash Frequency for Rural Multilane Highway Segments Procedure for safety prediction for a divided roadway segment: Apply Base Models, Apply CMFs, and calibration factor Nspf rd = e (a + b(ln(ADT)) + ln (L)) The SPF for expected average crash frequency for divided roadway segments on rural multilane highways is shown in Equation 11-9 and presented graphically in Figure 11-4: Nspf rd = e(a + b × In(AADT) + In(L)) (Equation 11-9) Where: Nspf rd = base total number of roadway segment crashes per year; AADT = annual average daily traffic (vehicles/day) on roadway segment; L = length of roadway segment (miles); and a, b = regression coefficients. Guidance on the estimation of traffic volumes for roadway segments for use in the SPFs is presented in Step 3 of the predictive method described in Section 11.4. The SPFs for undivided roadway segments on rural multilane highways are applicable to the AADT range from zero to 89,300 vehicles per day. Application to sites with AADTs substantially outside this range may not provide reliable results. The value of the overdispersion parameter is determined as a function of segment length as: (Equation 11-10) k = overdispersion parameter associated with the roadway segment; L = length of roadway segment (mi); and c = a regression coefficient used to determine the overdispersion parameter. Table 11-5 presents the values for the coefficients used in applying Equations 11-9 and 11-10. Npredicted rs = Nspf rd (CMF1r x CMF2r x CMFir)Cr Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Predicting Safety Performance of Rural Multilane Divided Highways Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Predicting Safety Performance of Rural Multilane Divided Highways Step #1 – Predict Crash Frequency: Nspf rd = e(a + b Ln ADT + Ln L) Instructor: From Chapter 11 of the highway safety manual. Using these regression coefficients for a & b to compute total crashes. Table 11-5 presents the values for the coefficients used in applying the SPF. C = the regression coefficient used to determine the overdispersion paarameter “k”. c = used to determine overdispersion parameter “k” for applying EB Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Base Conditions for Multilane Rural Divided Highway Segments Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Predicting Safety Performance of Rural Multilane Divided Highways Base Conditions for Multilane Rural Divided Highway Segments Baseline Geometric Conditions: Key Message:These are the nominal geometry or base conditions represented by the base model. Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Nspf rd = e(a + (b Ln AADT) + Ln L) Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Predicting Crash Frequency for Multilane Rural Divided Highways – Example Calculation: 4-lane Divided Rural Highway: AADT = 16,000 Length = 8.0 miles Nspf rd = e(a + (b Ln AADT) + Ln L) Example calculation for rural multilane divided highway for base conditions = e(-9.025 + 1.049 * Ln 16,000 + Ln 8.0) = e(3.2091) = 24.76 crashes per year Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Proportion of Crashes by Collision Type Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop Proportion of Crashes by Collision Type April 2009 The default proportions in Table 11-5 are used to break down the crash frequencies from Equation 11-9 into specific collision types. To do so, the user multiplies the crash frequency for a specific severity level from Equation 11-9 by the appropriate collision type proportion for that severity level from Table 11-6 to estimate the number of crashes for that collision type. Table 11-6 is intended to separate the predicted frequencies for total crashes (all severity levels combined), fatal-and-injury crashes, and fatal-and-injury crashes (with possible injuries excluded) into components by collision type. Table 11-6 cannot be used to separate predicted total crash frequencies into components by severity level. Ratios for property-damage-only (PDO) crashes are provided for application where the user has access to predictive models for that severity level. The default collision type proportions shown in Table 11-6 may be updated with local data. Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Applying Crash Modification Factors Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Applying Crash Modification Factors Npredicted rd = Nspf rd (CMF1 x CMF2 x ….) Npredicted rd = predicted number of crashes after treatment/improvement Nspf rd = base or existing number of crashes before treatment/improvement CMF = crash modification factor Recall the definition of CMFs -- An CMF less than 1 means a “positive effect” or benefit is expected Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

CMFs for Divided Highway Segments Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 CMFs for Divided Highway Segments Divided Highway Segments HSM Table 11-10 provides list of CMFs for Divided Highway Segments Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

CMF for Lane Width for Divided Rural Multilane Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 CMF for Lane Width for Divided Rural Multilane CMF1rd = (CMFRA -1.0) pRA + 1.0 CMF1rd—Lane Width on Divided Roadway Segments The CMF for lane width on divided segments is based on the work of Harkey et al. (3) and is determined as follows: CMFRA is determined from Table 11-16 based on the applicable lane width and traffic volume range. The relationships shown in Table 11-16 are illustrated in Figure 11-10. This effect represents 50 percent of the effect of lane width on rural two-lane roads shown in Chapter 10. The default value of pRA for use in Equation 11-16 is 0.50, which indicates that run-off-the-road, head-on, and sideswipe crashes typically represent 50 percent of total crashes. This default value may be updated based on local data. The SPF base condition for lane width is 12 ft. Where the lane widths on a roadway vary, the CMF is determined separately for the lane width in each direction of travel and the resulting CMFs are then averaged. Base condition is 12’ wide lane, pRA = 0.50 Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

CMF for Lane Width for Divided Rural Multilane Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 CMF for Lane Width for Divided Rural Multilane Example: for 11 foot lane and 18,000 ADT Instructor: Divided segments Example calculation for CMF1rd for a 11 ft lane CMF1rd = (CMFRA -1.0) pRA + 1.0 = (1.03 -1.0) 0.50 + 1.0 = (0.03) 0.50 + 1.0 = 1.015 Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

CMF for Shoulder Width and Shoulder Type (CMF2rd) for Divided Rural Multilane Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Base condition is 8’ wide shoulder; Effect of other shoulder types other than paved shoulders is unknown Instructor: From HSM, Chapter 11 CMF2rd—Right Shoulder Width on Divided Roadway Segments The CMF for right shoulder width on divided roadway segments was developed by Lord et al. (5) and is presented in Table 11-17. The SPF base condition for the right shoulder width variable is 8 ft. If the shoulder widths for the two directions of travel differ, the CMF is based on the average of the shoulder widths. The safety effects of shoulder widths wider than 8 ft are unknown, but it is recommended that a CMF of 1.00 be used in this case. The effects of unpaved right shoulders on divided roadway segments and of left (median) shoulders of any width or material are unknown. No CMFs are available for these cases. Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

CMF for Median Width (CMF3rd)for medians without Barrier Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Baseline: 30 ft median width Accounts for total crashes on segment Median width mainly affects median related crashes (20% of all crashes and cross-median crashes = 12% of all crashes on divided hwys) Instructor: From Chapter 11 of the highway safety manual CMF3rd—Median Width A CMF for median widths on divided roadway segments of rural multilane highways is presented in Table 11-18 based on the work of Harkey et al. (3). The median width of a divided highway is measured between the inside edges of the through travel lanes in the opposing direction of travel; thus, inside shoulder and turning lanes are included in the median width. The base condition for this CMF is a median width of 30 ft. The CMF applies to total crashes, but represents the effect of median width in reducing cross-median collisions; the CMF assumes that nonintersection collision types other than cross-median collisions are not affected by median width. The CMF in Table 11-18 has been adapted from the CMF in Table 13-9 based on the estimate by Harkey et al. (3) that cross-median collisions represent 12.2 percent of crashes on multilane divided highways. This CMF applies only to traversable medians without traffic barriers. The effect of traffic barriers on safety would be expected to be a function of the barrier type and offset, rather than the median width; however, the effects of these factors on safety have not been quantified. Until better information is available, a CMF value of 1.00 is used for medians with traffic barriers. Medians with traffic barriers: CMF = 1.0 Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

CMF for Lighting (CMF4rd ) for Divided Rural Multilane Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 CMF for Lighting (CMF4rd ) for Divided Rural Multilane CMF4rd = 1 – [(1– 0.72Pinr – 0.83Ppnr)Pnr] Instructor: From HSM, Chapter 11: CMF4rd—Lighting The SPF base condition for lighting is the absence of roadway segment lighting. The CMF for lighted roadway segments is determined, based on the work of Elvik and Vaa (1), as: CMF4rd = 1 – [(1 – 0.72 × pinr – 0.83 × ppnr) × pnr] (Equation 11-17) Where: CMF4rd = crash modification factor for the effect of lighting on total crashes; pinr = proportion of total nighttime crashes for unlighted roadway segments that involve a fatality or injury; ppnr = proportion of total nighttime crashes for unlighted roadway segments that involve property damage only; and pnr = proportion of total crashes for unlighted roadway segments that occur at night. This CMF applies to total roadway segment crashes. Table 11-19 presents default values for the nighttime crash proportions pinr, ppnr, and pnr. HSM users are encouraged to replace the estimates in Table 11-19 with locally derived values. HSM users are encouraged to replace the estimates in Exhibit 11-29 with locally derived values. = 1 – [(1– 0.72 x 0.323 – 0.83 x 0.677) x 0.426] = 0.912 * Base condition is no lighting present on the segment Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

CMF for Lighting (CMF4rd ) for Divided Rural Multilane Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 CMF for Lighting (CMF4rd ) for Divided Rural Multilane 13.13. Crash Effects of Highway Lighting 13.13.1. Background and Availability of CMFs Artificial lighting is often provided on roadway segments in urban and suburban areas. Lighting is also often provided at rural locations where road users may need to make a decision. 13.13.2. Highway Lighting Treatments with CMFs 13.13.2.1. Provide Highway Lighting Rural two-lane roads, rural multilane highways, freeways, expressways, and urban and suburban arterials The crash effects of providing highway lighting on roadway segments that previously had no lighting are shown in Table 13‑56. The base condition of the CMFs (i.e., the condition in which the CMF = 1.00) is the absence of lighting. The CMFs for nighttime injury crashes and nighttime crashes for all severity levels were derived by Harkey et al (15). using the results from Elvik and Vaa (8) along with information on the distribution of crashes by injury severity and time of day from Minnesota and Michigan. Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

= Total Crashes effect, CMF5rd = 0.94 Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 CMF for Automated Speed Enforcement (CMF5rd ) for Divided Rural Multilane Base condition is no Automated Speed Enforcement present CMF5rd = 1.00 Automated Speed Enforcement present; Injury crashes, CMF = 0.83 = Total Crashes effect, CMF5rd = 0.94 Instructor: CMF5rd—Automated Speed Enforcement Automated speed enforcement systems use video or photographic identification in conjunction with radar or lasers to detect speeding drivers. These systems automatically record vehicle identification information without the need for police officers at the scene. The SPF base condition for automated speed enforcement is that it is absent. Chapter 17 presents a CMF of 0.83 for the reduction of all types of fatal-and-injury crashes from implementation of automated speed enforcement. This CMF applies to roadway segments with fixed camera sites where the camera is always present or where drivers have no way of knowing whether the camera is present or not. Fatal-and-injury crashes constitute 37 percent of total crashes on rural multilane divided highway segments. No information is available on the effect of automated speed enforcement on noninjury crashes. With the conservative assumption that automated speed enforcement has no effect on noninjury crashes, the value of CMF5rd for automated speed enforcement would be 0.94 based on the injury crash proportion. Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Npredicted rd = Nspf rd (CMF1rd x CMF2rd x CMF3rd x CMF4rd x CMF5rd ) Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Applying CMFs to Predicted Crash Frequency for an Divided Rural Multilane Highway – Example: For Divided Rural Multilane Highway: 16,000 ADT, Length = 8.0 miles, 10 foot lanes, 6 ft paved shoulders, 25 foot median with no barrier, no lighting, no automated speed enforcement: Npredicted rd = Nspf rd (CMF1rd x CMF2rd x CMF3rd x CMF4rd x CMF5rd ) From Table 11-16, CMFra = 1.15 Instructor: Example calculation for selection of CMF’s for lane width, shoulder width, and shoulder type and then apply them to the base predicted crash number. CMF1rd = (CMFra -1.0) 0.50 + 1.0 = (1.15 -1.0) 0.50 + 1.0 = 1.075 Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Applying CMFs to Predicted Crash Frequency for an Divided Rural Multilane Highway – Example: For Divided Rural Multilane Highway: 16,000 ADT, Length = 8.0 miles, 10 foot lanes, 6 ft outside shoulders, 25 foot median with no barrier, no lighting, no automated speed enforcement: From Table 11-17, CMF2rd= 1.04 Instructor: Example calculation for selection of CMF’s for lane width, shoulder width, and shoulder type and then apply them to the base predicted crash number. Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

N = Nspf rd x CMF1rd x CMF2rd x CMF3rd x CMF4rd x CMF5rd Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Applying CMFs for Median Width, Lighting, and Auto Speed Enforcement – Example: For Divided Rural Multilane Highway: 16,000 ADT, Length = 8.0 miles, 10 foot lanes, 6 ft outside shoulders with 25 foot median with no barrier, no lighting, no automated speed enforcement: N = Nspf rd x CMF1rd x CMF2rd x CMF3rd x CMF4rd x CMF5rd CMF3rd from Table 11-18 (Median Width) = 1.00 25’ rounds to 30 foot median Instructor: Example calculation for selection of CMF’s for lane width, shoulder width, and shoulder type and then apply them to the base predicted crash number. CMF4rd from Table 11-19 (Lighting) = 1.00 CMF5rd (Automated Speed Enforcement) = 1.00 Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Nrd = Nspf rd x CMF1rd x CMF2rd x CMF3rd x CMF4rd x CMF5rd Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Applying CMFs to Predicted Crash Frequency for an Divided Rural Multilane Highway – Example: For Divided Rural Multilane Highway: 16,000 ADT, Length = 8.0 miles, 10 foot lanes, 6 ft paved shoulders, 25 foot median with no barrier, no lighting, no automated speed enforcement: CMF1rd = 1.075 CMF3rd = 1.00 CMF5rd = 1.00 CMF4rd = 1.00 CMF2rd = 1.040 Nrd = Nspf rd x CMF1rd x CMF2rd x CMF3rd x CMF4rd x CMF5rd Instructor: Example calculation for selection of CMF’s for lane width, shoulder width, and shoulder type and then apply them to the base predicted crash number. Nspf rd = 24.76 is from previous example calculation and CMFs are from appropriate Tables/Exhibits. = 24.76 x 1.075 x 1.040 x 1.00 x 1.00 x 1.00 = 27.68 crashes per year Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Applying CMFs to Predicted Crash Frequency for an Divided Rural Multilane Highways Additional CMF’s: Median Width Conversion Providing a Barrier Changing to a Less Rigid Roadside Barrier Use of Crash Cushions at Fixed Objects Use of Horizontal Alignment + Advisory Speed Signs Providing Rumble Strips Access Control Instructor: from HSM Chapter 13, there are several additional CMF applications that can be used to predict the crash frequency on Rural Multilane Roads. These will be presented in the new few slides. Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Changing Median Width on Rural Four-Lane Roads with Full Access Control Instructor: Chapter 13 of the Highway Safety Manual. 13.4.2.7. Change the Width of an Existing Median The main objective of widening medians is to reduce the frequency of severe cross-median collisions. Rural multilane highways and urban arterials Table 13‑12 through Table 13-16 present CMFs for changing the median width on divided roads with traversable medians. These CMFs are based on the work by Harkey et al. (15). Separate CMFs are provided for roads with TWLTLs, full access control and with partial or no access control. For urban arterials, the CMFs are also dependent upon whether the arterial has four lanes or more. The base condition of the CMFs (i.e., the condition in which the CMF = 1.00) is the presence of a 10-ft-wide traversable median. The type of traversable median (grass, depressed) was not identified. Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Median Width Conversion for Rural Multilane Highways with Partial or No Access Control Instructor: Chapter 13 of the Highway Safety Manual. 13.4.2.7. Change the Width of an Existing Median The main objective of widening medians is to reduce the frequency of severe cross-median collisions. Rural multilane highways and urban arterials Table 13‑12 through Table 13-16 present CMFs for changing the median width on divided roads with traversable medians. These CMFs are based on the work by Harkey et al. (15). Separate CMFs are provided for roads with TWLTLs, full access control and with partial or no access control. For urban arterials, the CMFs are also dependent upon whether the arterial has four lanes or more. The base condition of the CMFs (i.e., the condition in which the CMF = 1.00) is the presence of a 10-ft-wide traversable median. The type of traversable median (grass, depressed) was not identified. Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Installation of a Median Barrier for Rural Multilane Highways for 20,000 to 60,000 ADT Instructor: Chapter 13 of the draft Highway Safety Manual. Providing a median reduces crashes and injury crashes. On divided highways, median width includes the left shoulder. Medians may be depressed, raised, or flush with the road surface. 13.5.2.4. Install Median Barrier A median barrier is “a longitudinal barrier used to prevent an errant vehicle from crossing the highway median (8).” The AASHTO Roadside Design Guide provides performance requirements, placement guidelines, and structural andsafety characteristics of different median barrier systems (3). Rural multilane highways Installing any type of median barrier on rural multilane highways reduces fatal-and-injury crashes of all types, as shown in Table 13‑2 (8). The base condition of the CMFs (i.e., the condition in which the CMF = 1.00) is the absence of a median barrier. Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Crash Cushions at Fixed Roadside Features on Multilane Highways Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Crash Cushions at Fixed Roadside Features on Multilane Highways Instructor: 13.5.2.5. Install Crash Cushions at Fixed Roadside Features Rural two-lane roads, rural multilane highways, freeways, expressways, and urban and suburban arterials The crash effects of installing crash cushions at fixed roadside features are shown in Table 13‑24 (8). The crash effects for fatal and non-injury crashes with fixed objects are also shown in Table 13‑24 (12). The base condition of the CMFs (i.e., the condition in which the CMF = 1.00) is the absence of crash cushions. Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Install Continuous Shoulder Rumble Strips on Multilane Highways Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 Install Continuous Shoulder Rumble Strips on Multilane Highways 13.9.1. Background and Availability of CMFs Rumble strips warn drivers by creating vibration and noise when driven over. The objective of rumble strips is to reduce crashes caused by drowsy or inattentive drivers. In general, rumble strips are used in non-residential areas where the noise generated is unlikely to disturb adjacent residents. The decision to incorporate rumble strips may also depend on the presence of bicyclists on the roadway segment. Jurisdictions have not identified additional maintenance requirements with respect to rumble strips (23). The vibratory effects of rumble strips can be felt in snow and icy conditions and may act as a guide to drivers in inclement weather (13). Analysis of downstream crash data for shoulder rumble strips found migration and/or spillover of crashes to be unlikely (13). 13.9.2.1. Install Continuous Shoulder Rumble Strips Shoulder rumble strips are installed on a paved roadway shoulder near the travel lane. Shoulder rumble strips are made of a series of indented, milled, or raised elements intended to alert inattentive drivers, through vibration and sound, that their vehicles have left the roadway. On divided highways, shoulder rumble strips are typically installed on both the inner and outer shoulders (i.e., median and right shoulders) (28). The impact of shoulder rumble strips on motorcycles or bicyclists has not been quantified in terms of crash experience (29). Continuous shoulder rumble strips are applied with consistently small spacing between each groove (generally less than 1 ft). There are no gaps of smooth pavement longer than about 1 ft. Rural multilane highways The crash effects of installing continuous milled-in shoulder rumble strips on rural multi-lane divided highways with posted speeds of 55 to 70 mph are shown in Table 13‑44 (6). The crash effects on all types of injury severity and single-vehicle run-off-the-road crashes are also shown in Table 13‑44. The base condition of the CMFs (i.e., the condition in which the CMF = 1.00) is the absence of shoulder rumble strips. Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

CMF for Access Control for 4-Ln Divided Highways Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 CMF for Access Control for 4-Ln Divided Highways * From TTI synthesis CMFdd = (eb * (Dd - Dbase) -1.0) Ps + 1.0 Where: Dd = Driveway Density (Driveways per mile) Dbase = Base driveway density of 5 per mile b = coefficient Ps = subset proportion Instructor: From TTI synthesis (pg 3-33) - Uses AMF terminology and okay to replace with CMFs. Access Control This section is devoted to AMFs related to the access control elements of a rural highway. A review of the literature indicates that median type and driveway density have an influence on crash frequency. The correlation between median type and crashes is addressed previously in the section titled TWLTL Median Type. This section on access control describes the correlation between driveway density and crash frequency. The remaining three AMFs for driveway density were derived from safety prediction models developed by Vogt and Bared (3), Harwood et al. (7), and Wang et al. (4). The models by Vogt and Bared and by Harwood et al. are based on two-lane, undivided highway segments. The model by Wang et al. is based on four-lane, divided highways. The generalized AMF is shown below. Values of the variables b and Ps are provided in Table 3-26. Where: Dd = driveway density (driveways/mile) Dbase = Base Condition for driveways = 5 per mile Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

CMF for Access Control for 4-Ln Divided Highways: Example Safety and Operational Effects of Geometric Design Features for Multilane Rural Highways Workshop April 2009 CMF for Access Control for 4-Ln Divided Highways: Example For 4-Ln Divided, 32 driveways in 1.8 miles Driveway Density = 32/1.8 = 17.8 CMFdd = (eb(Dd - Dbase) -1.0) Ps + 1.0 Instructor: Example calculation for CMF for access CMFdd = (e0.034(17.8 - 5) -1.0) x 1.0 + 1.0 = 1.544 Session 2 – Predicting Highway Safety for Multilane Rural Highway Segments

Predicting Crash Frequency and CMFs for Rural Divided Multilane Highways Learning Outcomes: Described the models to Predict Crash Frequency for Divided Rural Multilane Highways Calculated Predicted Crash Frequency for Divided Rural Multilane Highways Described Crash Modification Factors Applied Crash Modification Factors Learning Objectives for Session #2 SPF models

Questions and Discussion: Predicting Crash Frequency and CMFs for Rural Divided Multilane Highways Questions and Discussion: